In 1916, Japan brought the American mosquitofish, Gambusia affinis, into the country as a way to control mosquitoes and the diseases they carried. The first introduction took place in Nara Prefecture after the fish arrived via Taiwan. More releases followed, including transfers to Okinawa and other parts of Japan. The fish were appealing as a mosquito-control tool because they readily eat mosquito larvae and can survive in shallow, slow-moving water. But the species did not stay within the places where people released it. It established itself in irrigation channels, ponds, rice fields and other waterways, and by 1970 it had spread to many parts of the country. Japan's National Institute for Environmental Studies lists the 1916 introduction, the later spread and the species' impact on native medaka, or Japanese rice fish.
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